http://iconpacks.mozdev.org/packs/firefox-world.html#c11
re: comment #10 . I have no idea why this could be happening. To install this icon pack manually, right-click the install link at the top of this page, save link as, and save the file somewhere. Then open it using a compression utility (eg. 'ark' if you're using KDE). Extract the 'default' folder inside to your Firefox chrome folder, for example, /usr/local/firefox/chrome or /usr/share/firefox/chrome or wherever it is. Note you'll very probably need to be logged in as root to do this, so run your compression utility as root, using an utility like 'kdesu' or from a terminal after becoming root using 'su'.
If you could let me know here if that works, it would be very helpful for sorting this problem out. Also, could you first try installing another icon pack on this website just by clicking and let me know if that works or if you get the same error message? Many thanks.
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8) Gecko/20051111 Firefox/1.5